Concurrency Issues. Past lectures: What about coordinated access across multiple objects? Today s lecture:
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1 Deadlock 1
2 Concurrency Issues Past lectures: Ø Problem: Safely coordinate access to shared resource Ø Solutions: Use semaphores, monitors, locks, condition variables Coordinate access within shared objects What about coordinated access across multiple objects? Ø If you are not careful, it can lead to deadlock Today s lecture: Ø What is deadlock? Ø How can we address deadlock? 2
3 Deadlocks Motivating Examples Two producer processes share a buffer but use a different protocol for accessing the buffers Producer1() { P(emptyBuffer) P(producerMutexLock) : } Producer2(){ P(producerMutexLock) P(emptyBuffer) : } A postscript interpreter and a visualization program compete for memory frames PS_Interpreter() { request(memory_frames, 10) <process file> request(frame_buffer, 1) <draw file on screen> } Visualize() { request(frame_buffer, 1) <display data> request(memory_frames, 20) <update display> } 3
4 Deadlock Definition Head Ready Running Tail ready queue Waiting Head Tail semaphore/ condition queues A set of processes is deadlocked when every process in the set is waiting for an event that can only be generated by some process in the set Starvation vs. deadlock Ø Starvation: threads wait indefinitely (e.g., because some other thread is using a resource) Ø Deadlock: circular waiting for resources Ø Deadlock è starvation, but not the other way 4
5 A Graph Theoretic Model of Deadlock The resource allocation graph (RAG) Basic components of any resource allocation problem Ø Processes and resources Model the state of a computer system as a directed graph Ø G = (V, E) Ø V = the set of vertices = {P 1,..., P n } {R 1,..., R m } P i R j Ø E = the set of edges = {edges from a resource to a process} {edges from a process to a resource} P i request edge R j allocation edge P k 5
6 Resource Allocation Graphs Examples A PostScript interpreter that is waiting for the frame buffer lock and a visualization process that is waiting for memory V = {PS interpret, visualization} {memory frames, frame buffer lock} Visualization Process Memory Frames PostScript Interpreter Frame Buffer 6
7 A Graph Theoretic Model of Deadlock Resource allocation graphs & deadlock Theorem: If a resource allocation graph does not contain a cycle then no processes are deadlocked A cycle in a RAG is a necessary condition for deadlock Is the existence of a cycle a sufficient condition? Game Visualization Process Memory Frames PostScript Interpreter Frame Buffer 7
8 A Graph Theoretic Model of Deadlock Resource allocation graphs & deadlock Theorem: If there is only a single unit of all resources then a set of processes are deadlocked iff there is a cycle in the resource allocation graph Memory Frames Visualization Process PostScript Interpreter Frame Buffer 8
9 Using the Theory An operational definition of deadlock Visualization Process Memory Frames PostScript Interpreter Frame Buffer A set of processes are deadlocked iff the following conditions hold simultaneously 1. Mutual exclusion is required for resource usage (serially useable) 2. A process is in a hold-and-wait state 3. Preemption of resource usage is not allowed 4. Circular waiting exists (a cycle exists in the RAG) 9
10 Dealing With Deadlock Deadlock prevention & avoidance Adopt some resource allocation protocol that ensures deadlock can never occur Ø Deadlock prevention/avoidance Guarantee that deadlock will never occur Generally breaks one of the following conditions: Mutex Hold-and-wait No preemption Circular wait *This is usually the weak link* Ø Deadlock detection and recovery Admit the possibility of deadlock occurring and periodically check for it On detecting deadlock, abort Breaks the no-preemption condition What does the RAG for a lock look like? 10
11 Deadlock Avoidance Resource Ordering Recall this situation. How can we avoid it? Producer1() { P(emptyBuffer) P(producerMutexLock) : } Producer2(){ P(producerMutexLock) P(emptyBuffer) : } Eliminate circular waiting by ordering all locks (or semaphores, or resoruces). All code grabs locks in a predefined order. Problems? Ø Maintaining global order is difficult, especially in a large project. Ø Global order can force a client to grab a lock earlier than it would like, tying up a resource for too long. Ø Deadlock is a global property, but lock manipulation is local. 11
12 Deadlock Detection & Recovery Recovering from deadlock R 1 R 2 R 3 R 4 P 1 P 2 P 3 P 4 P 5 Abort all deadlocked processes & reclaim their resources Abort one process at a time until all cycles in the RAG are eliminated Where to start? Ø Select low priority process Ø Processes with most allocation of resources Caveat: ensure that system is in consistent state (e.g., transactions) Optimization: Ø Checkpoint processes periodically; rollback processes to checkpointed state 12
13 Dealing With Deadlock Deadlock avoidance Banker s Algorithm Examine each resource request and determine whether or not granting the request can lead to deadlock Define a set of vectors and matrices that characterize the current state of all resources and processes Ø resource allocation state matrix Alloc ij = the number of units of resource j held by process i Ø maximum claim matrix Max ij = the maximum number of units of resource j that the process i will ever require simultaneously Ø available vector Avail j = the number of units of resource j that are unallocated P 1 P 2 P 3... P p R 1 R 2 R 3... R r n 1,1 n 1,2 n 1,3... n 1,r n 2,1 n 2,2 n 3,1... n p, n p,r <n 1, n 2, n 3,..., n r > 13
14 Dealing With Deadlock Deadlock detection & recovery What are some problems with the banker s algorithm? Ø Very slow O(n 2 m) Ø Too slow to run on every allocation. What else can we do? Deadlock prevention and avoidance: Ø Develop and use resource allocation mechanisms and protocols that prohibit deadlock Deadlock detection and recovery: Ø Let the system deadlock and then deal with it Detect that a set of processes are deadlocked Recover from the deadlock 14
DUH! Deadlocks. Concurrency Issues. The TENEX Case. If a process requests all systems buffers, operator console tries to print an error message
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